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United States Patent |
6,123,506
|
Brand
,   et al.
|
September 26, 2000
|
Diffuser pipe assembly
Abstract
The invention provides a diffuser assembly constructed of internal and
external concentrically nested bowl-shaped shells for directing an outward
flow of compressed air from a centrifugal compressor impeller to an
axially rearward diffused annular flow. The shells can be easily
manufactured from nested metal castings thereby eliminating much of the
cost and time involved in fabricating prior art diffusers of multiple
formed tubes brazed to a separately machined hub. The novel diffuser
assembly has two concentrically nested bowl-shaped shells, each shell
having an inner peripheral compressor impeller casing about a central
opening, and an outer edge. Opposing nested surfaces of the shells have an
array of mating grooves separated by abutting seam edges thus defining
individual diffuser ducts extending from the compressor impeller casings
to the outer shell edges when the shells are secured together.
Inventors:
|
Brand; Joseph Horace (Mississauga, CA);
Eleftheriou; Andreas (Woodbridge, CA)
|
Assignee:
|
Pratt & Whitney Canada Corp. (Longueuil, CA)
|
Appl. No.:
|
233023 |
Filed:
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January 20, 1999 |
Current U.S. Class: |
415/208.3; 415/211.1; 415/211.2; 415/224.5 |
Intern'l Class: |
F04D 029/44 |
Field of Search: |
415/208.3,208.2,211.2,211.1,224.5
|
References Cited
U.S. Patent Documents
2634685 | Apr., 1953 | Buchi.
| |
3333762 | Aug., 1967 | Vrana | 415/208.
|
4012166 | Mar., 1977 | Kaesser et al. | 415/208.
|
4854126 | Aug., 1989 | Chevis et al.
| |
Foreign Patent Documents |
967862 | Jul., 1949 | DE.
| |
467943 | Mar., 1969 | CH.
| |
673812 | Jun., 1952 | GB | 415/208.
|
Primary Examiner: Look; Edward K.
Assistant Examiner: Woo; Richard
Attorney, Agent or Firm: Astle; Jeffrey W.
Claims
What is claimed is:
1. A diffuser assembly for directing an outward flow of compressed air from
a centrifugal compressor impeller to an axially rearward diffused annular
flow, the diffuser assembly comprising:
internal and external concentrically nested bowl-shaped shells, each shell
having an inner peripheral compressor impeller casing about a central
opening, and an outer edge, opposing nested surfaces of the shells having
a plurality of mating grooves separated by abutting seam edges thus
defining a like plurality of individual diffuser ducts extending from the
compressor impeller casings to the outer shell edges when the shells are
secured together with fastening means.
2. A diffuser assembly according to claim 1, wherein the seam edges are
disposed on lands extending laterally between adjacent grooves.
3. A diffuser assembly according to claim 2, wherein the lands extend
continuously the length of the grooves.
4. A diffuser assembly according to claim 3, wherein the shells are of
substantially uniform thickness throughout.
5. A diffuser assembly according to claim 3, wherein the shells have
preselected zones of increased relative thickness.
6. A diffuser assembly according to claim 1, wherein mating seam edges of
each shell are secured together with fastening means selected from the
group consisting of: brazed surfaces; rivets; bolts; spot welds; and
continuously welded surfaces.
7. A diffuser assembly according to claim 1, wherein the grooves of each
shell have a cross-sectional area of increasing magnitude from the
compressor impeller casing to the shell outer edges.
8. A diffuser assembly according to claim 7, wherein the cross-sectional
area of a selected zone in the grooves of the internal shell is
substantially equal to the cross-sectional area of the adjacent zone in
the grooves of the nested external shell.
9. A diffuser assembly according to claim 7, wherein the grooves of each
shell have a substantially constant depth and a width of increasing
magnitude from the compressor impeller casing to the shell outer edges.
10. A diffuser assembly according to claim 9, wherein the grooves of each
shell have concave side walls of a selected radius thus defining diffuser
ducts with semicircular lateral profile when the shells are nested
together.
11. A diffuser assembly according to claim 1, wherein the shells comprise
metal castings.
12. A diffuser assembly according to claim 11, wherein the shells have
machined surfaces.
Description
TECHNICAL FIELD
The invention is directed to a diffuser for a gas turbine engine that is
simply constructed of two concentric nested shells, secured together by
brazing for example, each shell having opposing mating grooves which, when
the shells are nested together, define an array of diffuser ducts
extending from an inner peripheral compressor impeller casing to an
annular axially directed outer edge.
BACKGROUND OF THE ART
The compressor section of a gas turbine engine includes a diffuser
downstream of the centrifugal compressor turbines and impeller upstream of
the combustor. The function of a diffuser is to reduce the velocity of the
compressed air and simultaneously increase the static pressure thereby
preparing the air for entry into the combustor at a low velocity. High
pressure low velocity air presented to the combustor section is essential
for proper fuel mixing and efficient combustion.
The present invention is particularly applicable to gas turbine engines
which include a centrifugal impeller as the high pressure stage of the
compressor. Impellers are used generally in smaller gas turbine engines. A
compressor section may include axial or mixed flow compressor stages with
the centrifugal impeller as the high pressure section, or alternatively a
low pressure impeller and high pressure impeller may be joined in series.
A centrifugal compressor impeller draws air axially from a low diameter.
Rotation of the impeller increases the velocity of the air flow as the
input air is directed over impeller vanes to flow in a radially outward
direction under centrifugal force. In order to redirect the radial flow of
air exiting the impeller to an annular axial flow for presentation to the
combustor, a diffuser assembly is provided to redirect the air from radial
to axial flow and to reduce the velocity and increase static pressure.
A conventional diffuser assembly generally comprises a machined ring which
surrounds the periphery of the impeller for capturing the radial flow of
air and redirecting it through generally tangential orifices into an array
of diffuser tubes. The diffuser tubes are generally brazed or mechanically
connected to the ring and have an increasing cross-section rearwardly. As
a result, the narrow stream of air at high pressure taken into the
orifices in the ring are expanded in volume as the air travels axially
through the diffuser tubes. By the well known Bernoulli theorem (which
states that total energy of a fluid flow remains constant being the sum of
the pressure energy, potential energy and kinetic energy) the increase in
volume results in a reduced velocity and corresponding increase in static
pressure.
Fabrication of the diffuser tubes is extremely complex since they have a
flared internal pathway that curves from a generally radial tangential
direction to an axial rearward direction. Each tube must be manufactured
to close tolerances individually and then assembled to the machined
central ring. Complex tooling and labour intensive manufacturing
procedures result in a relatively high cost for preparation of the
diffusers.
In operation as well, diffusers often cause problems resulting from the
vibration of the individual diffuser tubes. To remedy vibration
difficulties, the diffuser tubes may be joined together or may be balanced
during maintenance procedures.
From an aerodynamic standpoint the joining of individual diffuser tubes to
the machined ring results in surface transitions which detrimentally
effect the efficiency of the engine. On the interior of the tube as it
joins the orifice in the ring, there is often a step or transition caused
by manufacturing tolerances in the assembly and brazing procedures. Since
the air in this section flows at extremely high velocity, the disturbance
in air flow and increase in drag as the air flows over inaccurately fit
transitions can result in very high losses in efficiency.
In general, the design of diffusers is not optimal since their complex
structure requires a compromise between the desired aerodynamic properties
and the practical limits of manufacturing procedures. For example, the
orifices in the impeller surrounding ring are limited in shape to
cylindrical bores or conical bores due to the limits of economical
drilling procedures. To provide elliptical holes for example, would
involve prohibitively high costs in preparation and quality control. The
shape of the diffuser pipes themselves is also limited by the practical
considerations of forming their complex geometry. In general, the diffuser
tubes are made in a conical shape and bent to their helical final shape
prior to brazing. Whether or not this conical configuration is optimal for
aerodynamic efficiency becomes secondary to the considerations of
economical manufacturing.
It is an aim of the invention therefore, to provide a diffuser assembly
which significantly reduces the tooling and manufacturing costs associated
with prior art diffuser assemblies.
It is a further aim of the invention to provide a diffuser assembly which
provides greater flexibility to the designers of gas turbine engines
enabling them to optimize the diffuser structure for improved aerodynamic
efficiency and vibration behaviour without concern for the manner in which
the diffuser will be actually manufactured.
It is a further aim of the invention to provide a diffuser assembly which
has shorter development time for new engines and considerably shorter lead
time in normal production by minimizing the operations required for
production.
It is a further aim of the invention to eliminate the internal transversal
steps between the diffuser tubes and separate internal machined ring of
the prior art.
It is a further aim of the invention to lower the weight of engines by
reducing the number of parts in a diffuser assembly, and using curved or
variable diffuser ducts to reduce the gas generator case diameter.
DISCLOSURE OF THE INVENTION
The invention provides a diffuser assembly constructed of internal and
external concentrically nested bowl-shaped shells for directing a radially
outward flow of compressed air from a centrifugal compressor to an axially
rearward diffused annular flow. The shells can be easily manufactured from
metal shapes, for example castings, thereby eliminating much of the cost
and time involved in fabricating prior art diffusers constructed of
multiple bent tubes brazed to a separately machined hub.
The novel diffuser assembly has two concentrically nested bowl-shaped
shells, each shell having an inner peripheral compressor impeller casing
about a central opening, and an outer edge. Opposing nested surfaces of
the shells have an array of mating grooves separated by abutting seam
edges thus defining individual diffuser ducts extending from the
compressor impeller casings to the outer shell edges when the shells are
secured together.
Preferably the seam edges are located on lands extending laterally between
adjacent grooves and the lands extend continuously the length of the
grooves. This construction reinforces the structure to resist vibration
through the diaphragm action of the lands which are preferably brazed
together throughout.
Several significant advantages result from this novel diffuser design. The
costs of production are reduced since tooling costs and manufacturing
complexity are dramatically reduced when only two shell parts are
required. Conventional diffusers in contrast require the separate
manufacture of several individual diffuser pipes, the machining of a
diffuser hub and precise fitting and brazing of the pipes to the hub.
Better performance results from elimination of the internal transversal
steps which are present in prior art diffusers at the joint between the
hub and each of the pipes.
The designer is freed from many of the constraints imposed by conventional
diffuser manufacturing techniques. To a large extent, conventional
diffuser configurations are dictated by the limitations of fabrication.
Many trade-offs between diffuser performance and manufacturing costs
compromise the efficiency of prior art diffusers.
The invention however, releases the designer from many of the
considerations dictated by prior art manufacturing methods. Using the
nested shells of the invention, the shape and cross-section of diffuser
ducts become completely independent of the manufacturing method used
permitting the diffuser duct shape to be optimised for aerodynamic and
structural efficiency.
By adoption of curved or variable diffusion diffuser ducts, the invention
can result in lower overall engine weight by reducing the gas generator
case diameter. In conventional engines, the diameter of the compressor
impeller combined with the outwardly disposed diffuser assembly largely
determines the gas generator case diameter. Any reduction in the outward
diameter of the diffuser assembly will reduce the gas generator case
diameter and lead to a smaller engine of lesser weight and reduced
external drag. The invention provides the designer with the freedom the
reduce the external diffuser diameter by curving the diffuser ducts
inwardly or by using variable cross-sectional profiles for the diffuser
ducts.
The thickness of diffuser duct walls can be optimised for improved
performance and minimum weight. If needed, reinforcement can be positioned
in selected zones of increased thickness or may include external
reinforcing ribs to control vibration, accommodate localised stresses or
resist wear.
Design changes can be incorporated with considerably shorter lead time and
development of new engines can proceed more rapidly. No tooling is needed
to produce prototype castings. Solid model data can be used with laser
photolithographic metal powder casting techniques to rapidly produce metal
prototypes for example.
Further details of the invention and its advantages will be apparent from
the detailed description and drawings included below.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be readily understood, one preferred
embodiment of the invention will be described by way of example, with
reference to the accompanying drawings wherein:
FIG. 1 is a perspective view of a diffuser assembly according to the
invention showing two bowl-shaped shells nested together to form an array
of diffuser ducts extending from a central compressor impeller casing to
axially directed exit nozzles at the outer edge of the diffuser assembly;
and
FIG. 2 is an exploded perspective view showing the internal and external
concentric shells of the diffuser assembly.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a diffuser assembly in accordance with the present invention
which directs an outward flow of compressed air from a centrifugal
compressor disposed within the internal opening to an axially rearward
diffused annular flow.
FIG. 2 shows an internal and external concentrically nested bowl-shaped
shell identified respectively with reference numerals 1 and 2. Each shell
1 and 2 has an inner peripheral compressor impeller casing 3 and 4 about a
relatively large central opening. When the shells 1 and 2 are nested
together as shown in FIG. 1, the casings 3 and 4 contain the outward flow
of air exiting from the periphery of the impeller, as it rotates at high
speed. Each shell 1 and 2 has an outer edge 5 and 6. As best indicated in
FIG. 1, the outward air flow contained within the impeller casings 3 and
4, exits through elongate nozzles formed along the outer edges 5 and 6 of
the nested shells 1 and 2.
To redirect and diffuse the air flow from a high pressure outwardly
directed flow from the impeller casings 3 and 4 to an axially rearwardly
directed flow passed the outer edges 5 and 6, each concentrically nested
shell 1 and 2 includes an array of mating grooves 7 and 8, which define
individual diffuser ducts when the shells 1, 2 are secured together with
fastening means (not visible).
In the embodiment shown, the grooves 7 and 8 are separated by abutting seam
edges 9 which are disposed on lands 10 extending laterally between
adjacent grooves 7 and 8. The lands 10 extend in the embodiment
illustrated continuously the length of grooves 7 and 8. The continuous
lands 10 join adjacent diffuser ducts together with a continuous diaphragm
which can be secured together with fastening means such as brazing,
riveting, bolting, spot welding, diffusion welding or fusion welding for
example.
It is anticipated by the inventors that the most economical manner of
producing these shells 1 or 2 is by metal casting and finish machining the
shells 1 and 2. The thickness of the shells 1 and 2 can be substantially
uniform throughout, or if desired for vibration control, structural
strength or wear resistance, the shells 1, 2 can easily be designed with
preselected zones of increased relative thickness.
As shown in FIG. 2 most clearly, the grooves 8 and 7 of each shell 1 and 2
have a cross-sectional area of increasing magnitude from the compressor
casing 3 and 4 to the shell outer edges 5 and 6. In the embodiment
illustrated, the seam edges 9 are disposed approximately in the center of
each diffuser duct and therefore the cross-sectional area of a selected
zone in the grooves 7 of the internal shell 1, are substantially equal to
the cross-sectional area of the adjacent zone in the grooves 8 of the
nested external shell 2. As well, in the illustrated embodiment, the
grooves 7 and 8 of each shell 1 and 2 have a substantially constant depth
with the width being of increasing magnitude from the compressor casings 3
and 4 to the shell outer edges 5 and 6. The grooves 7 and 8 of each shell
1 and 2, have concave side walls of a selected radius, and as indicated in
FIG. 1, the diffuser ducts defined therefore have a semi-circular lateral
profile when the shells are nested together.
It will be understood that the shape and orientation of the diffuser ducts
shown in the illustrated embodiment are by way of example only. A
significant advantage of the invention is to allow the designers to choose
any cross-section shape or path orientation for the diffuser ducts which
will optimize the efficiency of the diffuser assembly. A commonly used
diffuser pipe shape is the one shown in the drawings with a relatively
constant width and semi-circular rounded outer edges. However, that the
diffuser duct grooves 7 and 8 can as easily be made in an elliptical shape
or any other shape desired. Of particular advantage, the transition
between the impeller casings 3 and 4 and the grooves 7 and 8 can be made
completely smooth without the disadvantageous transition steps found in
the prior art. The shape of the grooves 7 and 8 immediately adjacent to
the casings 3 and 4 can be elliptical or any optimal shape determined by
designers.
As a result therefore, the novel dual shell diffuser assembly provided by
the invention significantly reduces the number of parts and tooling
required. Better vibration control and prediction results from the
structural integrity of the dual shell structure. Lower engine weight is
possible by using curved or variable diffusion diffuser ducts to reduce
the gas generator case diameter. Designers are free to quickly develop new
engines types with non-circular diffuser ducts if desired. Since fewer
operations are required in production, there is a considerably shorter
lead time required in producing diffuser assemblies. Better aerodynamic
performance will result from the elimination of internal transversal steps
present in the prior art between separate components of the diffuser
assembly.
Although the above description and accompanying drawings relate to a
specific preferred embodiment as presently contemplated by the inventors,
it will be understood that the invention in its broad aspect includes
mechanical and functional equivalents of the elements described and
illustrated.
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